Depending on the driving situation, a motor vehicle can be operated in traction mode, in which the motor vehicle is driven by the drive engine (internal combustion engine and/or electric motor), or it can be operated in overrun mode, in which the motor vehicle is driven by the vehicle wheels or coasts.
An overrun cut-off, at which no fuel injection is carried out in order to save fuel, occurs for a vehicle in overrun mode. In such an overrun mode, the vehicle drags the drive train. All dynamic parts of the drive train produce a drag torque, which counteracts the drive torque. The drag torque depends on the rotational speed of the internal combustion engine. That is, the lower the rotational speed of the internal combustion engine, the lower is also the drag torque. Therefore, for reduction of the drag torque in overrun mode, the gearbox control unit shifts the automatic transmission to a high gear, in which the internal combustion engine continues to rotate at reduced rotational speed. A generic drive device with such an overrun upshift is known, for example, from DE 10 2013 200 502 A1 or from DE 10 2010 041 324 A1.
The gear ratio steps that are available to the driving gears are usually dimensioned in such a way that a harmonic and smoothly drivable gear spacing to and gear engagement at the next higher driving gear results. Accordingly, the progressive ratio steps are to be laid out in a progressive ratio step diagram or in a gear spacing diagram. A progressive ratio step is defined as the quotient of the ratio of a driving gear to the ratio of the next-higher driving gear. In common practice, the progressive ratio step is greater the lower the driving gears are, and often gravitates toward the horizontal at high gears. The engine speed differential is therefore greater, the lower the driving gear is.
In the prior art, the ratio of the highest driving gear is also adjusted in an automatic transmission in agreement with the above design practice, with the automatic transmission being engaged as an overrun gear in overrun mode by the gearbox control unit and, namely, in such a way that the progressive ratio step from the second-highest driving gear to the highest driving gear (that is, the overrun gear) is small, so as to ensure a harmonic gear engagement in traction mode. In this case, however, on account of the small progressive ratio step from the second-highest driving gear to the highest driving gear (that is, overrun gear), the drag losses produced in the internal combustion engine continue to remain high.
A further reduction in the drag losses can be achieved by switching off the internal combustion engine in the overrun phases and disengaging the clutch between the internal combustion engine and the gearbox (transmission). In addition, the internal combustion engine must be restarted, as needed, even at higher speeds. This places an enormous load on the electrical system of the vehicle. For reasons of safety, therefore, a redundant power supply (that is, two batteries, which are decoupled from each other) is required. Moreover, during the overrun phase, diverse auxiliary assemblies—that is, the air-conditioning system, the vacuum-assisted braking system, and the like—are deactivated. In addition, if acceleration is to occur once again during the overrun phase, it takes a very long time for the internal combustion engine to connect to the gearbox once again in a force-locking manner.
In hybrid-driven motor vehicles having an internal combustion engine and a driving electric motor as second drive source, it is advantageous, for supplying the electrical system of the vehicle or for achieving an efficient recuperation operation, to engage the overrun mode and, if need be, a braking operation of the motor vehicle with as small a braking effect as possible by way of the internal combustion engine via the electric motor switched to operation as a generator. For this purpose, it is also appropriate in the case of a coupled internal combustion engine to shift the transmission into a higher driving gear.